All expressions in the Wolfram Language are ultimately built from a small number of distinct types of atomic elements.

AtomQ — test whether an expression is an atom

Head — find the head of any expression, including an atom

Symbols »

Symbol — the head of a symbol; create a symbol from a name

SymbolName  ▪  Context  ▪  Remove  ▪  Names  ▪  NameQ  ▪  ...

Strings »

String — the head of a string

StringQ  ▪  Characters  ▪  ToCharacterCode  ▪  StringLength  ▪  ToExpression  ▪  ...

Numbers »

Integer  ▪  Real  ▪  Rational  ▪  Complex

NumberQ — test for a number of any type

IntegerDigits  ▪  RealDigits  ▪  Precision  ▪  MachineNumberQ  ▪  ...